Every nuclear weapons program for decades has relied extensively on illicit imports of nuclear-related technologies. This book offers the most detailed public account of how states procure what they need to build nuclear weapons, what is currently being done to stop them, and how global efforts to prevent such trade could be strengthened. While illicit nuclear trade can never be stopped completely, effective steps to block illicit purchases of nuclear technology have sometimes succeeded in slowing nuclear weapons programs and increasing their costs, giving diplomacy more chance to work. Hence, this book argues, preventing illicit transfers wherever possible is a key element of an effective global non-proliferation strategy.